1. Field of the Invention
The invention relates to a cardiac stimulator with stimulation success monitoring, i.e., a cardiac stimulator such as an implantable heart pacemaker or an implantable cardioverter/defibrillator (ICD), which is independently capable of detecting the efficacy of a stimulation pulse delivered to the heart.
2. Description of the Related Art
Cardiac stimulators of the aforementioned type are already known in particular as implantable heart pacemakers. In the implanted and functional state, such heart pacemakers are usually connected by an electrode line of the aforementioned type to an electrode implanted in the heart and are designed to deliver electric stimulation pulses to the heart, or more specifically to individual ventricles of the heart, via the electrode. The stimulation pulses serve to energize the cardiac tissue (myocardium) of the respective ventricle and are delivered in particular when the heart does not contract naturally at the proper time, depending on the type of heart pacemaker. A contraction is induced by an electric pulse delivered to the myocardium. If the electric pulse is strong enough, it has the effect of locally depolarized the myocardial tissue and causing it to contract accordingly. The depolarization and contraction of the myocardial tissue should expand over the entire stimulated myocardium and should thus lead to the desired contraction of the corresponding ventricle of the heart.
The corresponding electric stimulation pulse must have a stimulation intensity above a respective stimulus threshold of the myocardial tissue. The stimulus threshold is a measure of the minimum stimulation intensity sufficient to induce depolarization of the myocardium and thus induce contraction of the respective ventricle of the heart. The stimulus threshold depends on various factors and is also variable over a period of time under some circumstances. In addition to the requirement of delivering a stimulation pulse of a sufficient stimulation intensity, there is the need to keep the energy to be expended for a stimulation pulse as low as possible. This energy is usually obtained from a heart pacemaker battery, which becomes depleted over a period of time. When this battery is depleted, the implanted heart pacemaker must be replaced (in a surgical procedure) by a new heart pacemaker. The longest possible operating time of the heart pacemaker and thus the longest possible battery lifetime are thus desirable. Furthermore, the energy for a stimulation pulse should also be as low as possible but still sufficient to stimulate the myocardium alone but not the surrounding muscle tissue.
There is thus a need, firstly, for the stimulation intensity of a stimulation pulse to be sufficient to trigger a contraction of the myocardial tissue. A higher stimulation intensity is associated with a higher energy consumption with influencing variables that are otherwise unchanged. The stimulation intensity depends firstly on the duration of the stimulation pulse and secondly on the intensity of the stimulation pulse. The intensity of a stimulation pulse in turn depends on the electric voltage with which a stimulation pulse is delivered to the myocardial tissue. A higher stimulation intensity therefore regularly also leads to a higher energy consumption. To achieve more reliable stimulation of the myocardial tissue, stimulation pulses which, from an energy standpoint, consume slightly more energy than would be necessary at a minimum are often delivered.
On the other hand, there is a need to minimize the energy consumption per stimulation pulse because this energy is derived from a heart pacemaker battery, which thereby becomes depleted.
For this reason, there are known cardiac stimulators with stimulation success monitoring. The stimulation success may be monitored each time a stimulation pulse is delivered (beat-to-beat) in order to re-administer a stronger stimulus in the event a stimulus is ineffective (automatic capture control, ACC). Or the stimulation success may be monitored as part of an automatic stimulus threshold test (automatic threshold test, ATT), in which stimulation pulses of differing intensity are triggered in a controlled manner to thereby determine the intensity above which a stimulation pulse is effective and thereby determine the respective stimulation threshold.
In all cases, the most reliable possible detection of a stimulation success, i.e., stimulated contraction of the respective myocardial tissue, is important.
In this context, it is known that measurement of the stimulus threshold (detection of effective stimulation) is performed on the stimulation electrode, thereby resulting in measurement uncertainties due to polarization artifacts, autoshort artifacts and blanking artifacts.
Furthermore, not all types of electrodes are suitable for this method, so the known method cannot be used to an unlimited extent.